Recent industrial tests of amorphous alloys under realistic working environments have indicated that the wear and corrosion resistances of this new category of alloys are at least one order of magnitude higher than that of conventional alloys currently in use. Other amorphous metal compounds are of interest as superconductors at low temperature and as magnetically soft alloys, etc.
Metallic glasses or, equivalently, amorphous metallic alloys can be formed by rapid solidification of liquid metals, or deposition of metallic vapors at rates sufficient to bypass crystallization. For the formation of a metallic glass, cooling rates in the range 10.sup.4 -10.sup.12 K/s are required to suppress nucleation and growth of more stable crystalline phases in undercooled alloy melts. These facts lead to severe restrictions in the synthesis of glassy metals. For example, simple heat transfer considerations require at least one of the specimen dimensions to be rather small, typically 10.mu.-100.mu..
Rapid solidification production methods all require a primary stage of generating and quenching the melt and, if necessary, a secondary stage of consolidating the product into a useful form. The primary stage requires rapidly bringing a melt of small cross-section into good contact with an effective heat sink. Several methods have been developed which can be classified as spray methods, chill methods and weld methods.
The spray techniques are preferable to the other methods since the cooling rate is rapid before, during and after solidification, increasing the likelihood of retaining the glassy micro-structure of the quenched, amorphous material. However, the spray methods are inefficient from an energy standpoint, provide very small sized product which must be further processed by consolidation or dispersed in a matrix resin to form a useful composite.
Recently, in patent application Ser. No. 462,441, filed Jan. 31, 1983, U.S. Pat. No. 4,564,396, the disclosure of which is expressly incorporated herein by reference, it was shown that much thicker and larger dimensional materials can be converted to contain metastable amorphous or fine crystalline phases in practical periods of several hours or less. The metastable phases are formed by diffusion of a first alloy component into a second component at a faster rate than the self-diffusion of the first component. Binary couples with high heats of mixing reduce their free energy by reacting to form a mixed amorphous phase. The method is practiced at a low temperature such that the diffusion and reaction to form the amorphous phase proceeds while the formation of crystalline compounds is kinetically bypassed. The method could be practiced by solid state reaction of polycrystalline diffusion couples in thin films, mixture of powders with nucleating agents or by diffusion of a gas into a layer of metal. In a related experiment, Koch et al [Appl. Phys. Lett. 43, 1017 (1983)] demonstrated that mechanical alloying of elemental nickel and niobium powders by high energy ball milling for extended times leads to the formation of an amorphous powder alloy.
Though these techniques result in the formation of larger amorphous particles or layers than are possible by quenching of melts, the particles are still limited in size. Furthermore, the diffusion technique cannot readily be practiced with large particles of the normal commercial size utilized for powder metallurgy since the surface to volume ratio of these powders is relatively small. Also, the natural surfaces of said powders may be inactive or have a kinetic barrier for reaction due to the presence of surface layers of metal oxide. Therefore, the temperatures required to initiate a reaction are too high to favor the growth of an amorphous phase.